The present invention relates to an optical isolator element used to eliminate a return light created upon introducing a light emitted from a light source to various optical elements and optical fibers, a method for producing such an element, and an optical isolator using such an element.
In an optical communication module, a light emitted from a light source such as a laser light source is incident on various optical elements and optical fibers. A part of the incident light is reflected and scattered by the end faces of the various optical elements and the optical fibers or inside them. A part of the reflected or scattered light returns to the light source as a return light if no measure is taken. Thus, an optical isolator is used in order to prevent the return light from returning to the light source.
Conventionally, an optical isolator of this type has been constructed such that a flat Faraday rotator is disposed between two polarizers and these three parts are accommodated in a tubular magnetic element via part holders. Normally, the Faraday rotator is adjusted to have such a thickness as to rotate a plane of polarization of a light having a specific wavelength by 45° in a saturated magnetic field, and the two polarizers are rotationally adjusted such that their transmitting and polarizing directions are shifted by 45° in rotating direction.
The optional isolator having the above construction requires three separate parts of the Faraday rotator and the two polarizers and the holders for the respective parts, resulting in a large number of parts and, thus, a large number of assembling steps. In addition, it has been cumbersome to give an optical adjustment between the respective parts, leading to a higher cost. Further, it has been difficult to miniaturize the optical isolator due to its large number of parts.
In view of this, there has been proposed an optical isolator designated for miniaturization by constructing an optical isolator element as an integral unit by adhering flat polarizers to both surfaces of a flat Faraday rotator by an adhesive and arranging the optical isolator element in a middle portion of a tubular magnetic element.
FIG. 10 is a diagram showing the construction of the optical isolator miniaturized by forming the parts into an integral unit by the adhesive.
In FIG. 10, an optical isolator 15 is comprised of an optical isolator element 20 in which a Faraday rotator 16 and polarizers 17, 18 are adhered by an optical adhesive 19 having a good light transmitting property and a controlled refractive index, and a tubular magnetic element 21 for accommodating the optical isolator element 20. Here, the polarizers 17, 18 have a function of absorbing light components polarized in one direction out of a propagating light and causing light components polarized in a direction orthogonal to the former light components to transmit. Further, the Faraday rotator 16 has a function of rotation a plane of polarization of a light having a specified wavelength by about 45° at a saturated magnetic field intensity. The two polarizers 17, 18 are arranged such that their transmitting and polarizing directions are shifted by about 45°.
FIGS. 11 to 14 are diagrams showing a method for producing the above conventional optical isolator element 20.
First, as shown in FIGS. 11 and 12, a polarizer base 22, a Faraday rotator base 23 and a polarizer base 24, which are large square optical elements having a side of about 10 mm, are formed into an integral unit by adhesion. Here, the transmitting and polarizing directions of the polarizer base 22 are set at a direction parallel with a certain side thereof (direction indicated by an arrow in FIG. 11), and those of the polarizer base 24 are set at a direction (direction indicated by an arrow in FIG. 11) at 45° to a certain side thereof. The polarizer bases 22, 24 and the Faraday rotator base 23 are so adhered that the respective sides thereof are parallel with each other, thereby obtaining an optical isolator element base 25. An optically transparent resin is used as the adhesive for adhering the respective optical elements as described above. Generally, an epoxy or acrylic organic adhesive is used as this adhesive.
Here, in the case that the optical isolator is required to have a high isolation, it is necessary to precisely adjust the rotational displacements of the polarizers bases 22, 24 at 45−α° with respect to a polarization rotation angle 45+α° of the Faraday rotator base 23. Specifically, the polarizer bases 22, 24 are so rotationally adjusted as to minimize the transmission of a light incident in reverse direction (for example, from the side of the polarizer base 24).
Next, as shown in FIGS. 13 and 14, the optical isolator element base 25 are out, for example, by a dicer into a multitude of optical isolator elements 20 in the form of small chips (see Japanese Unexamined Patent Publication No. H04-338916).
By using the method for producing a multitude of optical isolator elements 20 by forming a layered element by successively placing the polarizer base 22, the Faraday base 23 and the polarizer base 24, which are large optical element bases, one over another and cutting this layered element after the adhesion, operability can be improved, productivity can be increased and the number of parts can be reduced. Generally, an epoxy or acrylic organic adhesive is used as the optical adhesive 19 shown in FIG. 10.
The inventor of the present invention proposed a miniaturized optical isolator in which no resin was used to adhere the respective optical elements. In this optical isolator, a transparent low melting point glass was used to bond the Faraday rotator 16 and the polarizers 17, 18 forming the optical isolator element 20 shown in FIG. 10 instead of the resin. A method for producing this optical isolator element 20 is substantially same as the one shown in FIGS. 11 to 14 except that, upon forming the polarizer bases 22, 24 and the Faraday rotator base 23 into an integral unit, the transparent low melting point glasses are placed between the respective bases and heated up to a temperature where the low melting point glasses are molten, thereby bonding the respective bases (see Japanese Unexamined Patent Publication No. H08-146351).
However, the optical isolator element 20 obtained by integrally bonding the flat polarizers 17, 18 on both surfaces of the Faraday rotator 16 by the adhesive 19 as described above has a problem of a lower humidity resistance in the case that the adhesive 19 is an organic adhesive, whereby the use of this optical isolator element 20 particularly under a high-temperature and high-humidity condition is restricted. Further, there is a danger that the adhesive 19 changes its property when the optical isolator element 20 is used for a long time or used with a high-output laser beam, which leads to reduced reliability.
The optical isolator 15 including the optical isolator element 20 is exposed to high temperature upon being incorporated into a laser module. Thus, if an organic adhesive is used as the adhesive 19, the adhesive 19 is decomposed to produce air bubbles, thereby creating clearances between the Faraday rotator 16 and the polarizers 17, 18 or causing the parts of the optical isolator 15 to come off. Further, an outgas from the organic adhesive 19 attaches to the outer surfaces of optical components such as a laser chip and lenses to deteriorate optical characteristics.
In the optical isolator element 20 integrally bonded by adhering the flat polarizers 17, 18 on both surfaces of the Faraday rotator 16 by the low melting point glass 19, since the glass transition temperature of the low melting point glass 19 is as high as several hundreds degrees, a thermal stress increases upon being cooled to a room temperature after the glass 19 is molten to secure the respective parts, wherefore there is a danger of cracking the respective parts and the low melting point glass 19. Further, if a thermal stress acts on the Faraday rotator 16, the extinction ratio of the light transmitting the Faraday rotator 16 is reduced, thereby deteriorating various characteristics of the optical isolator 15, particularly a reverse-direction loss characteristic.